1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a method for accurately detecting defects in an IC using thermally induced frequency measurements of electrical signals.
2. Description of the Related Art
IC devices are formed from a die of active semiconductor devices. The die can be mounted in a hybrid circuit, printed circuit board (PCB), or a package. For environmental protection, the die may be covered by a passivation layer. However, a package is more typically used since it also dissipates heat and provides a lead system for electrical connections. There are many different types of packages including through-hole, surface mount device (SMD) dual/quad, and SMD area array packages.
FIG. 1 is a perspective view of a dual in-line package (DIP) (prior art). It is common for a package body or lead frame 100 to have a die attach area 102. The die 106 has electrical contact pads on its top surface. Inner leads 108 connect pads on die top surface to outer leads or lead frames 110. Once the inner leads are bonded to the lead frames, the package is sealed with ceramic, in a metal can, or in a polyimide. Epoxy resins are also a common choice. Glass beads are commonly mixed in with the epoxy to reduce strain in the epoxy film during changes in temperature.
Optical beam induced current (OBIC) is a semiconductor analysis technique performed using laser signal injection. The technique induces current flow in the semiconductor sample through the use of a laser light source. This technique is used in semiconductor failure analysis to locate buried diffusion regions, damaged junctions, and gate oxide shorts.
The OBIC technique may be used to detect the point at which a focused ion beam (FIB) milling operation in bulk silicon of an IC must be terminated. This is accomplished by using a laser to induce a photocurrent in the silicon, while simultaneously monitoring the magnitude of the photocurrent by connecting an ammeter to the device's power and ground. As the bulk silicon is thinned, the photocurrent increases as the depletion region of the well to substrate junction is reached. FIB milling operations are terminated in a region below the well depth, so the device remains operational.
Thermally Induced Voltage Alteration (TIVA) laser imaging techniques can also be used to electrically detect optical transmission through thin packages. The enclosed integrated circuit acts as a detector while the laser is scanned on the outside of the package. In one aspect, the TIVA laser causes some heating of the IC and the resultant resistance change is detected electronically through the leads of the device. The OBIC laser causes the generation of electron-hole pairs in the crystal silicon die that results in a current that is then detected as a voltage at the pins of the device. In some aspects, both lasers are used simultaneously.
FIG. 2 depicts an exemplary system for detecting optical paths through an IC package (prior art). Selected power pins from the IC are connected to a sense amp 300, as are the IC grounds. A laser 302 scans an area of an IC package 304 overlying the die (not shown). The scanning area is defined by an x-y coordinate system. The IC package 304 is mounted on a movable table 306. Alternately, the IC package position is fixed and the laser moves. The scan pattern need not necessarily follow the x-y grid. In some aspects, only selected areas of the package surface over the die are scanned.
FIG. 3 is a detailed schematic of a sense amplifier (prior art). The sense amplifier 400 connects lines V+ and S+ to IC power supply lines, while inputs V− and S− are typically connected to ground. In other aspects, the sense amplifier lines may be connected to signal inputs or signal outputs. Imaging is performed through the top of the package. Electrical connections are made to the power supply pins of the device and those connections go to a current amplifier for video imaging the package surface as the OBIC laser is scanned. The OBIC laser has a 1065 nanometers wavelength and does not ablate the epoxy mold compound. The optical path (OBIC or TIVA) testing lasers typically use a power of less than 100 milliwatts.
However, using OBIC and TIVA techniques, it is difficult to distinguish between defect and normally operating circuitry. That is, there are many sites that are thermally sensitive and produce a response and shows up in images. Not all of these sites are defects. It is not adequate to compare the image of a good reference unit to the unit under test, because a defect can induce connected non-defective circuit elements to behave differently than the corresponding circuitry in the reference unit.
Typically, destructive physical analysis follows the TIVA analysis and the results of the TIVA analysis are used to determine the direction in which the destructive physical analysis should proceed. For example, time consuming and destructive techniques such as mechanical micro-probing or cross-sectioning may be employed. If multiple sites are indicated, and the diagnosis is ambiguous, then the physical analysis may inadvertently destroy the true failure site.
An analysis of noise (frequency) provides more information than simple voltage and current analysis. To that end, the light emissions from an integrated circuit can be collected using a low light sensitive camera or discrete light defector (Zheng et al., “A Novel Fault Isolation Technique Using Noise Detection and Characterization of Light Emitted from Integrated Circuits”, Proceedings from the 24th International Symposium for Testing and Failure Analysis, Nov. 15-19, 1998, pp. 467-471). However, it is not possible to access every potential defect region so that optical data can be collected. Further, while the TIVA method suffers from the above-mentioned limits, it has the advantage of using a laser to heat the sample as a means of inducing defect measurement. The optical method provides no means of using temperature to enhance noise measurements.
It would be advantageous if a method existed to more positively differentiate defect sites from non-defect sites in an IC, without resulting to physical analysis or probing.
It would be advantageous if a means existed for using electrical signal noise (frequency) analysis to identify IC defect regions. It would be advantageous if a laser could be used, inducing noise measurements with high temperatures.